Bigger or smaller?
Oppel–Kundt illusion
In each row, which space is longer: the empty one or the one filled with marks?
They are the same length. Filled spaces look longer.
Why? The marks give your eye more things to count along the way, and your brain takes “more things” to mean “more distance”.
Picture: IBSGR, CC BY-SA 4.0 · Wikipedia(opens in a new tab)